1. Field of the Invention
This invention relates to lock stitch sewing machines employing a bobbin on which under or locking thread is wound for concatination with loops of needle thread in the formation of lock stitches. More particularly this invention relates to a novel and advantageously reliable system for monitoring the supply of thread remaining on the bobbin and indicating to the machine operator impending bobbin thread exhaustion.
2. Description of the Prior Art
Low bobbin thread signaling devices are known, as disclosed, for instance, in U.S. Pat. No. 4,178,866, Dec. 18, 1979 of Adams in which reception of a beam of light directed tangentially of the bobbin hub is used to signal low bobbin thread when the bobbin thread is unwound sufficiently to pass the light beam. A problem with such signaling devices is the inability to distinguish from reception of ambient light with resulting frequent false alarms.
The U.S. Pat. No. 4,413,581, Nov. 8, 1983 of Logan discloses an optical low bobbin thread detecting system in which the beam of light tangential to the bobbin hub is generated only in predetermined phases by an LED which is controlled by an oscillator and driver, and an arrangement is provided in order to monitor light reception which is registered only in matching relationship with that of light generated by the LED. Although the phase locked loop does provide a high degree of protection against false signalling as result of ambient light it does involve the expense of LED, oscillator and driver dedicated specifically to low bobbin thread detection, and it is subject to unreliability due to fluctuations in the power supply.